This invention is based on Patent Application No. 306191/1998 filed on Oct. 27, 1998 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a locating method of an optical sensor, as well as an adjustment method of dot printing positions using the optical sensor located by the method and a printing apparatus. An adjustment of dot printing position is, for example, registration of dot printing positions where printing is performed in the bi-directions of a forward scan and a reverse scan and a printing registration between the heads where printing is performed using a plurality of print heads.
2. Description of the Related Art
The optical sensor which has a light-emitting portion and a light receiving section to irradiate with the light on a measuring object to detect the reflected light and to obtain some information concerning the object is applied to various apparatuses in various fields.
In a field of a printing technique, the sensor is used, for example, on the occasion of printing processing, in order to detect existence of the printing medium which is a printing object and the size (e.g. paper width) thereof. In addition, in order to perform a processing (so-called head shading) which corrects a drive condition of a printing element for obtaining a printing image without unevenness in density, it is sometimes used as a means which reads the unevenness in density which is developed on a predetermined test patterns. In addition, it is effective to use the optical sensor in a printing registration, and hereinafter, a processing of the registration for dot printing positions is described.
In recent years, the office automation instruments such as the personal computer and the word processor which is relatively cheap are widely used, and an improvement in high-speed technique and an improvement in high image quality technique of various recording apparatuses for printing-out the information which are entered by the instruments are developed rapidly. In recording apparatuses, a serial printer using a dot matrix recording (printing) method comes to attention as a recording apparatus (a printing apparatus) which realizes printing of a high speed or high image quality with the low cost. For such printers, as the technique which prints at high speed, for example there is a bi-directional printing method and as the technique which the prints in high image quality, for example, there is a multi scanning printing method.
(Bi-directional Printing Method)
As the improvement in high-speed technique, in a printing head which has a plurality of printing elements, although it is also thought to plan an increase in the number of a printing elements and an improvement in a scanning speed of the print head, it is also an effective method to perform bi-directional printing scannings of the print head.
Although, since there is usually the time required for paper-feeding and paper-discharging or the like, it does not become a simply proportional relation, in the bi-directional printing a printing speed of approximately two times can be obtained as compared with the one-directional printing in the printing apparatus.
For example, when using the print head which the 64 pieces of ejection openings are arranged with 360 dpi (dots/inch) in printing density in a direction different from the printing scanning (main scanning) direction (for example, in a sub-scanning direction which is the feeding direction of the printing medium), a printing is performed on, a printing medium of A4 size set in the direction of the length, the printing can be completed by scanning of approximately 60 times. The reason is that, in one-directional printing, each printing scanning is performed only at the time of the movement in the one direction from the predetermined scanning commencement position, and since non-printing scanning to the inverse direction for returning to the scanning commencement position from a scanning completion position is attended, reciprocation of approximately 60 times is required. On the other hand, printing is completed by the reciprocating printing scanning of approximately 30 times in bi-directional printing, so that printing can be performed and since it becomes possible on at the speed of approximately 2 times, whereby bi-directional printing can be considered to be an effective method for an improvement in a printing speed.
In order to register dot-forming positions (for example, for an ink jet printing apparatus, a deposition or landing position of ink) at a forward trip and a return trip together in such bi-directional printing, using a position detection means such as an encoder, based on the detecting position, printing timing is controlled. However, it has been thought that since to form such a feedback controlled system causes an increase in the cost of the printing apparatus, it is difficult to realize this, in the printing apparatus which is relatively cheap.
(Multi Scanning Printing Method)
Secondly, a multi scanning printing method is explained as one example of the improvement in high image quality technique.
When printing is performed using the print head which has a plurality of printing elements, quality of the printed image depends on performance of a print head itself greatly. For example, in the case of the ink-jet print head, the slight differences, which is generated in a print head manufacturing step, such as variations of a form of ink ejection openings and the elements for generating energy for ejecting ink such as an electro-thermal converting elements (ejection heaters), influence a direction and an amount of ejected ink, and result in the cause which makes the unevenness in density of the image which is formed finally to reduce the image quality.
Specific examples are described using FIGS. 1A to 1C and FIGS. 2A to 2C. Referring to FIG. 1A, a reference numeral 201 denotes a print head, and for simplicity, is constituted by the eight pieces of nozzles 202 (herein, as far as not mentioned specifically, refer to the ejection opening, the liquid passage communicated with this opening and the element for generating an energy used for ink, in summary). A reference numeral 203 denotes the ink, for example, which are ejected as a drop from the nozzle 202. It is ideal that the ink is ejected from each ejection opening by the approximately uniform amount of discharge and in the justified direction as shown in this drawings. When such discharge is performed, as shown in FIG. 1B, ink dots which are justified in size are deposited or landed on the printing medium and, as shown in FIG. 1C, the uniform images that there is no unevenness in density also as a whole can be obtained.
However, there are the variations in the nozzles in the print head 20 actually as is mentioned above, and when printing is performed as mentioned above as it is, as shown in FIG. 2A, the variations are caused in size of the ink drops and in the ejecting direction of ink discharged from nozzles and the ink drops are deposit or landed on a printing medium as shown in FIG. 2B. In this drawing, a part of the white paper that an area factor cannot be served up to 100% periodically exists with respect to the horizontal scanning direction of the head, moreover, in contrast with this, the dots are overlapped each other more than required or white stripes as shown in the center of this drawing have been generated. A gathering of the landed dots in such condition forms the density distribution shown in FIG. 2C to the direction in which nozzles are arranged, and the result is that, so far as usually seen by eyes of a human, these objects are sensed as the unevenness in density.
Therefore, as a countermeasure of this unevenness in density, the following method has been devised. The method is described using FIGS. 3A to 3C and FIGS. 4A to 4B.
According to this method, in order that the printing with regard to the same region as shown in FIG. 1B and FIG. 2B is made to be completed, the print head 201 is scanned 3 times as shown in FIG. 3A and FIGS. 4A to 4C. The region defining four pixels which is a half of eight pixels as a unit in the direction of length in the drawing has been completed by two passes. In this case, the 8 nozzles of the print head are divided into a group of 4 nozzles of upper half and 4 nozzles of lower half in the drawing and the dots which one nozzle forms by scanning of one time are the dots that the image data are thinned into approximately a half in accordance with the certain predetermined image data arrangement. Moreover, at the second scanning, the dots are embedded in the image data of the half of the remaining and the regions defined four pixels as the unit are completed progressively. Hereinafter, the printing method described above is referred to as a multi scanning printing method.
Using such printing method, even when the print head 201 which is equal to the print head 201 shown in FIG. 2A are used, the influence to the printed image by the variations of each nozzle is reduced by half, whereby the printed image becomes as shown in FIG. 3B and no black stripe and white stripe as shown in FIG. 2B becomes easy to be seen. Therefore, the unevenness in density is fairly also mitigated as compared with the case of FIG. 2C as shown in FIG. 3C.
When such printing is performed, although at first scanning and at second scanning, the image data are mutually divided in a manner to complement each other in accordance with the certain predetermined arrangement (a mask), usually, this image data arrangement (the thinned patterns) as shown in FIGS. 4A to 4C, at every one pixel arranged in rows and columns, it is most general to use the formation which forms a checker or lattice matrix.
In a unit printing region (here, per four pixels), printing is completed by the first scanning which forms the dots into the checker or lattice pattern and the second scanning which forms the dots into the inverted checker or lattice pattern.
Moreover, usually, travel (vertical scanning travel) of the printing medium between each main scanning is established at a constant, and in the case of FIG. 3 and FIG. 4, is made to move every four nozzles equally.
(Dot Alignment)
As an example of the other improvement in high image quality technique in the dot matrix printing method, there is a dot alignment technique adjusting the dot depositing position. A dot alignment is an adjustment method adjusting the positions which the dots on the printing medium have formed by any means, and in general, the prior dot alignment has been performed as follows.
For example, a ruled line or the like is printed on a printing medium in depositing registration of the forward scan and the reverse scan upon reciprocal or bi-directional printing by adjusting printing timing in the forward scan and the reverse scan respectively, while a relative printing position condition in reciprocal scan is varied. The results of printing has been observed by a user oneself to select the printing condition where best printing registration is achieved, that is, the condition that printing is performed without offset of the ruled line or the like and to set the condition directly into the printing apparatus by entering through a key-operation or the like or to set the depositing position condition into the printing apparatus by operating a host computer through an application.
Moreover, the ruled line or the like is printed on the medium under printing in the printing apparatus having a plurality of heads, when printing is performed between a plurality of heads, while a relative printing position condition between a plurality of heads is varied, with the respective head. As is mentioned above, the optimum condition that best printing registration is achieved has been selected to vary the relative printing position condition to set the printing position condition into the printing apparatus every each head in the mentioned-above manner.
Here, the case where the offset of the dots has been occurred is described.
(Problems Upon Performing Image-formation by Bi-directional Printing)
Due to bi-directional printing, the following problems has been caused.
First, when the ruled line (the ruled line of the longitudinal direction) in the direction perpendicular to the horizontal scan of the print head is printed, between the ruled line element which is printed in the forward scan and the ruled line element which is printed in the reverse scan, the dot depositing positions are not registered and the ruled line is not formed into a straight line, but a difference in level occurs. This is referred to as a so-called xe2x80x9coffset in ruled linexe2x80x9d, and this is considered to be the most general disorder which can be recognized by the usual users. In the many cases, the ruled line is formed by a black color, whereby, though the offset in ruled line has been understood as the problem where a monochrome image is formed generally, a similar phenomenon can be caused in the color image also.
When multi scanning printing is used along with bi-directional printing in order to achieve high image quality, even though in bi-directional printing the depositing positions are not registered, as an effect of the multi scanning printing, the offset in the pixel level is not easily seen, but from a macroscopic viewpoint, the entire image can be seen as being unequal and is recognized as an unpleasant figure by the user. This is generally called a texture, and appears on the image in the specific period where there is the offset in the delicate depositing position. In an image with strong contrast, such as a monochrome image, it can easily be seen, and for a printing medium capable of high-density printing, such as a coat paper where middle-tones printing is performed, it can easily be seen. (Problems in the case of performing the image formation using a plurality of the print heads).
In the printing apparatus having a plurality of heads, the problems of the case where the offset in the depositing positions of the dots between a plurality of heads has been occurred is discussed.
When the image printing is performed, several colors are combined to perform the image formation frequently, and it is general to use four colors which added black in addition to three primary colors of yellow, magenta and cyan and it is used most abundantly. When in the case where a plurality of print heads for printing these colors are used, there is the offset of the depositing positions between the print heads, depending upon the amount of the offset, when a different color one another is about to be printed on the same pixel, a deviation in color matching is caused. For example, magenta and cyan are used to form the blue image, and although the part that the dots of both colors are overlapped becomes blue, the part which is not overlapped each other does not become blue, so that the deviation in color matching (irregular color) that each independent color tone appears is caused. When this occurs partially, it does not become easy to be seen, but when this phenomenon occurs in the direction of scanning continuously, a band-shaped deviation in color matching with a certain specific width is caused, so that the image becomes unequal. In addition, in a region adjacent the image region in the case of in the regions of the same color, when there is no offset in the depositing positions of the dots, a uniform impression and color development differ between the image regions adjacent each other, so that the image that there is a sense of incongruity as the image is formed. Moreover, though this deviation in color matching does not become easy to be seen in the case of an ordinary paper, it becomes easy to be seen, when a favorable printing medium in color development such as a coat paper is used.
Moreover, in the case where a different color is printed on adjoining the pixel, when there is the offset in the depositing positions of the dot, the clearance, that is, the region which is not covered by the ink on the part have caused and, the ground of the printing medium can be seen. This phenomenon frequently is called xe2x80x9cwhite clearancexe2x80x9d, since the case of a white ground is frequent in the printing medium generally. This phenomenon is easy to be seen in the image high in contrast, and when a black image is formed as a colored back ground, the white clearance which no ink is deposited between a black and coloration, since a contrast between white and black is high, can be easy to be seen more clearly.
It is effective to perform the above-mentioned dot alignment in order to suppressed occurring of the problems as mentioned above. However, the complicatedness that the user should observe the results which the depositing registration conditions are varied by the eyes to select the optimized the depositing registration condition to perform entering operations is accompanied, and moreover, since fundamentally, a judgment for obtaining the optimum printing position by observing through eyes is forced on the user, the establishment which is not optimized can be set. Therefore, it is especially unfavorable to the user who is not accustomed to operation.
Moreover, the user is required to spend at least twice as much time and effort since the user should print the image to perform the depositing registration and in addition, to perform conditional establishment after observing the image to judge the image quality. Therefore, to obtain an apparatus or a system excellent in operability is disadvantageous from the viewpoint of time-consumption.
Namely, it has been desired strongly that the apparatus or system capable of printing the image at a high speed and of the high-quality image without occurring the problem on the image formation as above-mentioned and the problem on the operability is realized at a low cost by designing to be able to register the depositing position without using a feedback controlling means such as an encoder by an opened loop.
An object of the present invention is to provide a dot alignment method excellent in operability at a low cost. Moreover, other objects of the present invention are to detect optical characteristics of a printed image and calculate registration conditions of optimum dot alignment based on the detected result so as to automatically establish the registration conditions basically without forcing a user into judgement or registration, and further, to eliminate the influence of the factors of fluctuations caused by printing in detecting the optical characteristics so as to stably perform the detection.
Additionally, a further object of the present invention is to provide a method for appropriately determining location of an optical sensor.
In a first aspect of the present invention, there is provided an optical sensor locating method comprising the steps of:
providing a light-emitting portion for irradiating an object to be measured with light and a photosensing portion for sensing the light reflected on the object to be measured; and
locating the optical sensor at a position where an output characteristic of the optical sensor is substantially stable with respect to factors of fluctuations at the time of measurement relatively to the object to be measured.
In a second aspect of the present invention, there is provided a printing registration method for processing for performing printing registration in a first printing and a second printing with respective to a printing apparatus for performing printing of an image by the first printing and the second printing with predetermined conditions of a dot forming position on a printing medium by using a printing head, the method comprising the steps of:
pattern-forming for controlling the printing head to form a plurality of patterns respectively having optical characteristics corresponding to a plurality of shifting amounts, the plurality of patterns being respectively formed corresponding to the plurality of shifting amounts of relative printing positions of the first printing and the second printing, the patterns being formed by the first printing and the second printing;
measuring respective optical characteristics of the plurality of the formed patterns by using an optical sensor providing a light-emitting portion for irradiating the pattern as an object to be measured with light and a photosensing portion for sensing the light reflected on the object to be measured; the optical sensor being located at a position where an output characteristic of the optical sensor is substantially stable with respect to factors of fluctuations at the time of measurement relatively to the object to be measured; and
acquiring an adjustment value of a dot forming position condition between the first printing and the second printing on the basis of respective optical characteristics of the plurality of patterns measured.
In a third aspect of the present invention, there is provided a printing apparatus for performing printing of an image on a printing medium by using a printing head, comprising:
means for measuring by using an optical sensor providing a light-emitting portion for irradiating an object to be measured with light and a photosensing portion for sensing the light reflected on the object to be measured; and
means for supporting the optical on a position where an output characteristic of the optical sensor is substantially stable with respect to factors of fluctuations at the time of measurement relatively to the object to be measured.
In a fourth aspect of the present invention, there is provided a printing apparatus for performing printing of an image by a first printing and a second printing with predetermined conditions of a dot forming position on a printing medium by using a printing head, comprising:
pattern-forming means for controlling the printing head to form a plurality of patterns respectively having optical characteristics corresponding to a plurality of shifting amounts, the plurality of patterns being respectively formed corresponding to the plurality of shifting amounts of relative printing positions of the first printing and the second printing, the patterns being formed by the first printing and the second printing;
means for measuring respective optical characteristics of the plurality of the formed patterns by using an optical sensor providing a light-emitting portion for irradiating the pattern as an object to be measured with light and a photosensing portion for sensing the light reflected on the object to be measured; the optical sensor being located at a position where an output characteristic of the optical sensor is substantially stable with respect to factors of fluctuations at the time of measurement relatively to the object to be measured; and
means for acquiring an adjustment value of a dot forming position condition between the first printing and the second printing on the basis of respective optical characteristics of the plurality of patterns measured.
In a fifth aspect of the present invention, there is provided a printing system provided with a printing apparatus for performing printing of an image by a first printing and a second printing with predetermined conditions of a dot forming position on a printing medium by using a printing head, and a host apparatus for supplying an image data to the printing apparatus, comprising:
pattern-forming means for controlling the printing head to form a plurality of patterns respectively having optical characteristics corresponding to a plurality of shifting amounts, the plurality of patterns being respectively formed corresponding to the plurality of shifting amounts of relative printing positions of the first printing and the second printing, the patterns being formed by the first printing and the second printing;
means for measuring respective optical characteristics of the plurality of the formed patterns by using an optical sensor providing a light-emitting portion for irradiating the pattern as an object to be measured with light and a photosensing portion for sensing the light reflected on the object to be measured; the optical sensor being located at a position where an output characteristic of the optical sensor is substantially stable with respect to factors of fluctuations at the time of measurement relatively to the object to be measured; and
means for acquiring an adjustment value of a dot forming position condition between the first printing and the second printing on the basis of respective optical characteristics of the plurality of patterns measured.
Incidentally, hereafter, the word xe2x80x9cprintxe2x80x9d (hereinafter, referred to as xe2x80x9crecordxe2x80x9d also) represents not only forming of significant information, such as characters, graphic image or the like but also represent to form image, patterns and the like on the printing medium irrespective whether it is significant or not and whether the formed image elicited to be visually perceptible or not, in broad sense, and further includes the case where the medium is processed.
Here, the wording xe2x80x9cprinting mediumxe2x80x9d represents not only paper to typically used in the printing apparatus but also cloth, plastic film, metal plate and the like and any substance which can accept the ink in broad sense.
Furthermore, the wording xe2x80x9cinkxe2x80x9d has to be understood in broad sense similarly to the definition of xe2x80x9cprintxe2x80x9d and should include any liquid to be used for formation of image patterns and the like or for processing of the printing medium.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.